Method and apparatus for electrochemical processing

ABSTRACT

A method of electrochemically processing an article having a semi-insulating element and a conducting element comprises forming a conductive layer on the article, the conductive layer comprising at least a first region covering at least a portion of the semi-insulating element and a second region covering at least a portion of the conducting element; gripping with at least one conductive gripper a portion of the first region; submerging at least a portion of the second region in an electrolyte while keeping the conductive gripper out of said electrolyte; and conducting current through a circuit comprising the conducting element, the conductive layer, the conductive gripper, and an electrode submerged in the electrolyte. The conductive layer is preferably formed by diffusing a dopant from a vapor phase.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of International Application No.PCT/US03/09596 filed Mar. 28, 2003, the entirety of which isincorporated herein by reference. This application claims the benefit ofU.S. Provisional Application 60/368,717, filed Mar. 29, 2002, and U.S.Provisional Application 60/368,543 filed Mar. 29, 2002, the entiretiesof which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention is directed to a method of electrochemicallyprocessing a conducting element which is positioned on a semi-insulatingelement. In preferred aspects, the present invention is directed to amethod of electrochemically processing a conducting epitaxial layer (orlayers) which have been grown on a semi-insulating substrate.

BACKGROUND OF THE INVENTION

There are a variety of devices known to those of skill in the art whichrequire one or more structure including a conducting element positionedon a semi-insulating element, e.g., a conducting layer formedepitaxially on a semi-insulating substrate. In addition, there are avariety of situations known to those of skill in the art where it isnecessary or desirable to electrochemically process such a conductingelement positioned on a semi-insulating element.

For instance, growth of epitaxial layers on semi-insulating substrateshas been used to provide series connection of semiconductor devices(e.g., in high-voltage, low-current solar cells and thermophotovoltaiccells). Electrochemical processing of such devices can significantlysimplify their fabrication.

For samples having a conducting epitaxial layer on a semi-insulatingsubstrate, two methods of electrochemical processing which have beenemployed are: (i) holding the sample 10 (see FIG. 1), which includes aconducting epitaxial layer 11 and a semi-insulating substrate 12, with ametal holder 13 contacting an electrolyte 14 and applying currentthrough a circuit including an electrode 15 submerged in the electrolyte14, the electrolyte 14, the conducting layer 11 and wires 16 and 17, and(ii) a similar arrangement except for holding the sample with a metalholder which is not in contact with the electrolyte (see FIG. 2). Bothof these methods have substantial drawbacks. Method (i) leads to theuncertainty of the current density for the processed area because anunknown portion of current flows through the low-resistance metalholder. For method (ii), the processed area is considerably limited toavoid contacting the holder with the electrolyte. In method (ii), afactor which limits the area which can be processed is the electrolytecreeping along the processed surface toward the holder, a phenomenonwhich is especially strong for rough surfaces and long processes.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a more precise and more reproduciblemethod of performing electrochemical processing on a conducting elementpositioned on a semi-insulating element.

In accordance with the present invention, there is provided a method ofelectrochemical processing, comprising:

-   -   forming a conductive layer on the outside of an article        comprising a semi-insulating element and a conducting element;    -   gripping the article with a conductive gripper at a region of        the conductive layer which is adjacent to the semi-insulating        element;    -   submerging the portion of the conductive layer which is in        contact with the conducting element in an electrolyte while        keeping the conductive gripper out of the electrolyte; and    -   conducting current through a circuit comprising the conducting        element, the conductive layer, the conductive gripper, and an        electrode submerged in the electrolyte.

Preferably, the article is a wafer, including a semi-insulatingsubstrate having substantially flat front and back surfaces and aconducting epitaxial layer grown on the substrate and also havingsubstantially flat front and back surfaces, whereby the gripper can beplaced on the back surface of the wafer (i.e., the portion of theconductive layer adjacent to the back of the substrate) and only thefront side of the wafer (i.e., the portion of the conductive layeradjacent to the front surface of the conducting epitaxial layer)contacts the electrolyte, such that the processed area or currentdensity can be specified with high accuracy.

A particularly preferred method for forming a conductive layer on theoutside of the article comprising a semi-insulating element and aconducting element is to perform a diffusion of a dopant suitable forthe top epitaxial layer from a vapor phase. Using a vapor phase dopant,the dopant can penetrate the article from all sides, including thesemi-insulating element.

The present invention is also directed to devices utilizing the anodicoxide layer thus grown, including electrical and optoelectrical devicessuch as transistors, capacitors, waveguides, light emitters, lightdetectors and lasers, e.g., as disclosed in U.S. Pat. Nos. 5,696,023,5,567,980, 5,550,081 and 5,262,360 (the entireties of which are herebyincorporated herein by reference), as well as methods of making suchdevices including methods according to the present invention. Forexample, the present invention is directed to forming a III-V MOS deviceon a semi-insulating substrate using an anodic oxide, e.g., in forming ahigh-speed, high-power device (such as cell phones and radars).

In addition, the present invention relates to the masking andpassivation of semiconductors utilizing the anodic oxide that forms fromthe practice of the present invention.

The invention may be more fully understood with reference to theaccompanying drawings and the following detailed description of theinvention.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 is a schematic view depicting a first known method ofelectrochemically processing a sample having a conducting epitaxiallayer on a semi-insulating substrate.

FIG. 2 is a schematic view depicting a second known method ofelectrochemically processing a sample having a conducting epitaxiallayer on a semi-insulating substrate.

FIG. 3 is a schematic view depicting a method of electrochemicallyprocessing an article comprising a conducting element on asemi-insulating element.

DETAILED DESCRIPTION OF THE INVENTION

As noted above, the present invention is direct to a method ofelectrochemically processing an article comprising a conducting elementpositioned on a semi-insulating element.

The present invention can be used to perform any kind of electrochemicalprocessing, a wide variety of which are well known to those of skill inthe art. For example, the present invention can be used to performelectroplating. Also, the present invention can be used to performanodic oxidation. The present invention can be used to make galvaniccontacts. Similarly, the present invention can be used to perform anyother kind of electrochemical processing.

The present invention is applicable to electrochemically processing anyarticle comprising any conducting element positioned on anysemi-insulating element. The present invention is therefore not limitedto any particular kind of conducting element made of any particularmaterial, or any particular kind of semi-insulating element made of anyparticular material. Those of skill in the art will appreciate thatskilled artisans would readily be able to apply the methods of thepresent invention to any such article.

Standard solar cells generate relatively high current but relatively lowvoltage (usually less than 1 volt), e.g., voltage which is not highenough to charge standard batteries (which typically require at leastseveral volts). To increase voltage, a solar array consisting of severalcells can be connected in series. For such series connection, insulatingsemiconductor substrates can be used. Because trenches are providedbetween respective cells, the presence of a conductive layer inaccordance with the present invention, which conductive layerelectrically connects the opposite surfaces of each cell, does notpresent an electrical problem.

A first side of the conducting element is in contact with a secondsurface of the semi-insulating element. Preferably, the second surfaceand a first surface of the semi-insulating element are substantiallyflat, with the first surface being substantially parallel to the secondsurface. Preferably, the first side and a second side of the conductingelement are substantially flat, with the first side being substantiallyparallel to the second side.

As noted above, the method according to the present invention comprisesforming a conductive layer on an outside of the article comprising asemi-insulating element and a conducting element.

The conductive layer comprises at least a first region and a secondregion, the first region covering at least a portion of thesemi-insulating element, the second region covering at least a portionof the conducting element.

Preferably, the conductive layer completely surrounds the article (i.e.,is formed on all of the outer surfaces of the conductive layer). It isnot necessary, however, for the conductive layer to completely surroundthe article, so long as the conductive layer has a least a first regioncovering at least a portion of the semi-insulating element and a secondregion covering at least a portion of the conducting element, and thefirst region and the second region are electrically connected to eachother (e.g., the first region and the second region both being connectedto a third region of the conductive layer).

Any suitable method for forming a conductive layer on the outside of thearticle can be employed. A wide variety of techniques for forming aconductive layer on the outside of an article comprising asemi-insulating element and a conducting element are known to those ofskill in the art, and any such technique can be employed.

As noted above, a preferred method for forming the conductive layer onthe outside of the article is to perform a diffusion of a dopantsuitable for the top epitaxial layer from a vapor phase. Using a vaporphase dopant, the dopant can penetrate the article from all sides,including the semi-insulating element, such that the conductive layercompletely surrounds the article. A preferred dopant in such a method iszinc. The present invention is not limited, however, to any particularmethod of forming the conductive layer, or any particular type ofconductive layer.

Next, a portion of the first region is gripped with at least oneconductive gripper. A variety of conductive grippers are known to thoseof skill in the art, and any such gripper can be employed in carryingout the present invention. For example, an example of a suitable gripperis metal vacuum tweezers.

At least a portion of the second region of the conductive layer (i.e.,the portion which covers at least a portion of the conductive element)is submerged in an electrolyte while keeping the conductive gripper outof the electrolyte.

Preferably, the article is submerged to a depth where the entireconducting element is beneath the level of the electrolyte and theentire semi-insulating element is above the level of the electrolyte.

Any suitable electrolyte can be employed, depending on the nature of theelectrochemical processing being performed. Those of skill in the artcan readily select an appropriate electrolyte based on the nature of theelectrochemical processing being performed and the chemical nature ofthe other elements involved.

Current is then conducted through a circuit comprising the conductivelayer, the conductive gripper, and an electrode submerged in theelectrolyte. Any suitable electrode can be employed, and persons ofskill in the art are readily able to select an appropriate electrode,based on the nature of the electrochemical processing being carried outand the materials of the other elements. Likewise, any suitablecircuitry can be employed in order to provide the necessary current forperforming the electrochemical processing being performed, and personsof skill can readily select appropriate circuitry. The present inventionis not limited to any particular electrolyte, circuitry or electrodes.

Referring to FIG. 3, there is shown an article 30 (including aconducting epitaxial element 31 and a semi-insulating element 32), and aconductive layer 33 surrounding the article 30 on all sides. The article30 is held with a pair of metal vacuum tweezers 34 and is submerged inan electrolyte 35, to a depth where the conducting epitaxial element 31is beneath the level of the electrolyte 35 and the semi-insulatingelement 32 is above the level of the electrolyte 35. Current is appliedthrough a circuit including an electrode 36 which is submerged in theelectrolyte 35, the electrolyte 35, the conducting epitaxial element 31,the conductive layer 33 and wires 37 and 38.

For example, in a specific embodiment of the present invention, there isprovided a process for making a photovoltaic cell for use in a devicecomprising scintillating fibers and a solar cell array, in which thescintillating fibers are used to absorb short wavelength light andre-emit the light at a longer wavelength, and the solar cell array isused to absorb the re-emitted light and convert it into energy.

According to this process, a gallium-arsenide semi-insulating layer (orany other semi-insulating layer, e.g., indium phosphide) is provided,and then an n-doped gallium arsenide layer is epitaxially formed on thesemi-insulating layer (using any material which acts as an n-dopant,e.g., tin dopant), then a p-doped gallium arsenide layer is epitaxiallyformed on the n-doped layer (using any material which acts as ap-dopant, e.g., germanium dopant), and then an un-doped aluminum galliumarsenide layer is formed on the p-doped layer. Preferably, the n-dopedlayer, the p-doped layer and the un-doped layer are formed by liquidphase epitaxy, preferably in a single melt run. Preferably, the un-dopedaluminum gallium arsenide layer has a composition at or nearAl_(0.8)Ga_(0.2)As. Next, a low-temperature vapor phase zinc diffusionis carried out, e.g., in a pseudo-closed graphite cassette. Thisdiffusion leads to the formation of a thin zinc-doped diffused layer onthe outer surfaces of the entire wafer, including the semi-insulatingsubstrate.

Then the zinc-doped aluminum gallium arsenide layer is anodicallyoxidized to form an anti-reflection coating. This anodic oxidation iscarried out in accordance with the present invention, with the galliumarsenide semi-insulating layer functioning as the semi-insulatingelement, the aluminum gallium arsenide layer functioning as theconducting element, and the zinc-doped diffused layer surrounding theentire wafer functioning as the conductive layer. In the resultingproduct, the aluminum gallium arsenide layer functions as a passivatinglayer.

In another aspect of the present invention, an important trend insemiconductor technology is the use of Group III-V materials for thefabrication of semiconductor devices. While the utilization of silicon(Si) is still prevalent, Group III-v compounds—such as GaAs—have beenthe subject of much research due to significant advantages thesecompounds offer. For example, Group III-V compounds generally exhibitlarger band gaps, larger electron mobilities and have the ability toproduce light, which properties result in unique electrical and opticalcharacteristics.

Notwithstanding these qualities, there has been difficulty in producing,on the Group III-v semiconductor, an oxide layer of desired thicknessthat exhibits the necessary surface state and electrical propertiesrequired for practical application. In this regard, the oxide must beable to fulfill, without the disruption and strain caused byover-expansion of the oxide thickness, a variety of functions in apractical and consistent manner. Examples of these functions include:serving as a mask during device fabrication, providing surfacepassivation, isolating one device from another (dielectric isolation, asopposed to junction isolation), acting as a component in the anatomy ofvarious device structures and providing electrical isolation ofmultilevel metallization systems. Accordingly, the presence of ahigh-quality, stable oxide layer having adequate physical properties andproper thickness, as provided by the present invention, is important tothe successful development of Group III-V semiconductor technology.

Silicon-based materials, unlike Group III-V semiconductors, readily forma high quality oxide (SiO₂) by such methods as reacting the siliconcrystal with water vapor, e.g., in the form of steam. Indeed, the veryexistence of silicon-based integrated circuit technology is largely dueand owing to this ability of silicon to form a high quality siliconoxide. Moreover, this oxide is a native oxide, as opposed to a depositedoxide layer. Native oxides, e.g., the anodic oxides formed according tothe present invention, are more desirable than deposited oxides in thatthey are monolithic with the crystal and thus avoid potentialmismatching of dielectric characteristics and problems associated withoxide-substrate interface bonding, such as lifting and cracking.Further, deposition processes are on the whole more complicated andcostly than are methods of growing a native oxide thus making the lattermore attractive for commercial use.

1. A method of electrochemically processing an article comprising aconducting element positioned on a semi-insulating element, the methodcomprising: forming a conductive layer on an article, said articlecomprising a semi-insulating element and a conducting element, saidconductive layer comprising at least a first region and a second region,said first region covering at least a portion of said semi-insulatingelement, said second region covering at least a portion of saidconducting element; gripping with at least one conductive gripper aportion of said first region; submerging at least a portion of saidsecond region in an electrolyte while keeping said conductive gripperout of said electrolyte; and conducting current through a circuitcomprising said conducting element, said conductive layer, saidconductive gripper, and an electrode submerged in said electrolyte.
 2. Amethod as recited in claim 1, wherein said semi-insulating element has asubstantially flat first surface and a substantially flat secondsurface, said first surface of said semi-insulating element beingsubstantially parallel to said second surface of said semi-insulatingelement, and said conducting element is substantially flat and has asubstantially flat first side and a substantially flat second side, saidfirst side of said conducting element being substantially parallel tosaid second side of said conducting element, said first side of saidconducting element being in contact with said second surface of saidsemi-insulating element.
 3. A method as recited in claim 1, wherein saidconductive layer completely surrounds said article.
 4. A method asrecited in claim 1, wherein said conductive layer is formed by diffusinga dopant from a vapor phase.
 5. A method as recited in claim 1, whereinsaid article is submerged in said electrolyte to a depth wheresubstantially an entirety of said conducting element is beneath a topsurface of said electrolyte and substantially an entirety of saidsemi-insulating element is above said top surface of said electrolyte.